This invention relates to a new method for preparing divalent-europium-activated calcium sulfide phosphors.
In the past, rare-earth-activated CaS (calcium sulfide) phosphors were prepared by one of the following three methods: (1) the flux method in which a mixture of calcium sulfate and a suitable rare-earth compound is reduced with carbon in sodium sulfate or other suitable flux; (2) the solid-state-reaction method in which a mixture of sulfides is reacted in a sulfurizing atmosphere such as sulfur vapor or hydrogen sulfide, or (3) the direct-sulfurization method in which a mixture of carbonates or/and sulfates is reacted with hydrogen sulfide. In these methods, strontium and/or barium may be substituted for at least part of the calcium.
In the case of the flux method, a fairly large amount of alkali is inevitably incorporated in CaS crystals, and anion impurities are reported to have little effect on luminescent properties. In the case of the solid-state reaction and direct-sulfurization methods, the incorporation of other anions, such as P, As, and halogen, is reported to improve luminescent properties. In particular, the efficiency of a CaS phosphor containing 0.1 mole % Eu and 0.005 mole % Ce (with respect to Ca) is reported to be improved as much as 30% by incorporating 0.2 to 1 mole % Cl and 0.005 to 0.23 mole % P. The body color of phosphors that contain optimum Eu concentrations (0.05 to 0.1 mole %) is brownish yellow in the case of the flux method, and is light pink in the case of phosphors prepared by the solid-state reaction and direct-sulfurization methods.